Photothermal multi-species detection in a hollow-core fiber with frequency-division multiplexing

A photothermal gas sensor has been developed for simultaneous multi-species detection in a hollow-core fiber based on the frequency-division multiplexing technique. By passing multiple pump lasers and a probe laser through a gas-filled hollow-core anti-resonant fiber (HC-ARF), the absorption-generated modulation of the refractive index at various frequencies is sensitively detected by an in-line interferometer. Three distributed-feedback diode lasers at C-band, L-band and U-band are employed as pump lasers for C₂H₂, CO₂ and CH₄ detection. Assisted by optical power amplifiers, the sensor achieves noise equivalent concentrations of 2.5 ppb, 21 ppm and 200 ppb for C₂H₂, CO₂ and CH₄, respectively. The sensor demonstrates excellent linear response to gas concentrations with R-square values between 0.996 and 0.999. Measurement of the three gas species with time-varying concentrations, as well as the aforementioned experiments, further validates the simultaneous multi-species detection with high sensitivity, compact size, and extremely low gas consumption.